1. Field of the Invention
The present invention generally relates to frequency assignment, reuse and/or sharing among communications systems having both a terrestrial and a satellite component (dual-mode) and, more particularly, to a satellite-terrestrial communications system and method of operation thereof that provides frequency assignment, reuse and/or sharing between the respective portions of the satellite system and/or terrestrial underlay systems associated therewith, while substantially reducing interference therebetween.
2. Background Description
In satellite-terrestrial systems that reuse the same spectrum, there is a need to efficiently allocate at least a portion of the frequency spectrum of, for example, a first satellite coverage area to, for example, a terrestrial network associated with a terrestrial coverage area. The present invention provides a system and method for efficiently assigning, reusing and/or sharing the spectrum between satellite and/or terrestrial systems in a manner that facilitates efficient frequency spectrum usage, while minimizing interference between the respective satellite and terrestrial systems. The present invention can also be applied to multiple satellite systems as well, in addition to, or instead of, terrestrial systems.
FIG. 1 shows a prior art satellite radiotelephone system, as shown in U.S. Pat. No. 6,052,586, incorporated herein by reference. As shown in FIG. 1, a satellite radiotelephone system includes a fixed satellite radiotelephone system 110 and a mobile satellite radiotelephone system 130. The fixed satellite radiotelephone system 110 uses a first satellite 112 to communicate with a plurality of fixed radiotelephones 114a, 114b and 114c in a first communication area 116.
Fixed satellite radiotelephone communication system 110 communicates with the plurality of fixed radiotelephones 114a–114c using a first air interface 118 (e.g., at C-band). Control of the fixed satellite system 110 may be implemented by a feeder link 122 which communicates with a gateway 124 and the public switched (wire) telephone network (PSTN) 126.
The feeder link 122 may include communication channels for voice and data communications, and control channels. The control channels are indicated by dashed lines in FIG. 1. The control channels may be used to implement direct communications between fixed radiotelephones, as shown for example between radiotelephones 114a and 114b. The control channels may also be used to effect communications between a fixed satellite radiotelephone 114c and a mobile radiotelephone or a wire telephone via gateway 124 and PSTN 126. The feeder link 122 may use the same air interface or a different air interface from the first air interface 118.
Still referring to FIG. 1, mobile satellite radiotelephone system 130 includes a second satellite 132 that communicates with a plurality of mobile radiotelephones 134a–134d which are located in a second communication area 136. Mobile satellite radiotelephone system 130 communicates with mobile radiotelephones 134 using a second air interface 138 (e.g., at L-band or S-band). Alternatively, the second air interface 138 may be the same as the first air interface 118. However, the frequency bands associated with the two air interfaces will generally be different.
A feeder link 142 may be used to communicate with other satellite, cellular or wire telephone systems via gateway 144 and PSTN 126. As with fixed satellite system 110, the feeder link 142 may include communication channels shown in solid lines and control channels shown in dashed lines. The control channels may be used to establish direct mobile-to-mobile communications, for example, between mobile radiotelephones 134b and 134c. The control channels may also be used to establish communications between mobile phones 134a and 134d and other satellite, mobile or wire telephone systems.
As with the fixed satellite radiotelephone system 110, the mobile satellite radiotelephone system 130 may employ more than one satellite 132 and will generally communicate with large numbers of mobile radiotelephones 134. The fixed and mobile satellite radiotelephone system may also use a common satellite.
Still referring to FIG. 1, a congested area may be present in the mobile satellite radiotelephone system 130 where a large number of mobile radiotelephones 134e–134i are present. As is also shown in FIG. 1, this congested area may be in an overlapping area 128 between first communication area 116 and second communication area 136. If this is the case, excess capacity from fixed satellite radiotelephone system 110 may be offloaded to mobile satellite radiotelephone system 130.
Capacity offload may be provided by at least one fixed retransmitting station 150a, 150b, that retransmits communications between the fixed satellite radiotelephone system 110 and at least one of the mobile radiotelephones. For example, as shown in FIG. 1, first fixed retransmitting station 150a retransmits communications between satellite 112 and mobile radiotelephones 134e and 134f. Second fixed transmitting station 150b retransmits communications between the satellite 112 and mobile radiotelephones 134g, 134h and 134i. The fixed retransmitting station need not be located in an overlapping area as long as it can retransmit communications between the fixed satellite radiotelephone system in the first area, and the mobile radiotelephones.
The fixed retransmitting stations communicate with the satellite 112 using first air interface 118. However they communicate with the mobile radiotelephones using the second air interface 138. Accordingly, from the standpoint of the mobile radiotelephones 134e–134i, communication is transparent. In other words, it is not apparent to the mobile radiotelephones 134e–134i, or the users thereof, that communications are occurring with the fixed satellite radiotelephone system 110 rather than with the mobile satellite radiotelephone system 130. However, additional capacity for the mobile satellite radiotelephone system 130 in the congested areas adjacent the fixed retransmitting stations 150 may be provided.
As shown in FIG. 1, a mobile radiotelephone can establish a communications link via the facilities of the fixed satellite radiotelephone system, even though the mobile radiotelephone is designed, manufactured and sold as a terminal intended for use with the mobile satellite radiotelephone system. One or more operators may offer both mobile and fixed telecommunications services over an overlapping geographic area using two separate transponders in separate satellites or within the same “hybrid” satellite, with one transponder supporting mobile satellite radiotelephones and the other supporting fixed satellite radiotelephones. As capacity “hot spots” or congestion develops within certain spot beams of the mobile radiotelephone system, the fixed system, with its much higher capacity, can deploy fixed retransmitting stations to relieve the capacity load of the mobile system.
FIG. 2A shows a seven-cell frequency reuse pattern which may be used by the mobile satellite radiotelephone system 130. Within each of the relatively large mobile system cells, each typically being on the order of 400–600 kilometers in diameter, frequencies used by adjacent cells may be locally retransmitted by the retransmitting station at reduced, non-interfering power levels, and reused as shown in FIGS. 2B and 2C, thus substantially increasing the effective local capacity.
Accordingly, fixed retransmitting stations, located within the fixed system's footprint or coverage area, receive signals from the fixed satellite and retransmit these signals locally. Frequency translation to bring the signals within the mobile system's frequency band will generally be provided. In the reverse direction, the fixed retransmitting stations receive signals from mobile radiotelephones and retransmit signals from the mobile radiotelephones to the fixed satellite system. Again, frequency translation to bring the signals within the fixed system's frequency band will generally be provided.
The mobile radiotelephones are ordinarily used with the mobile satellite system. Accordingly, the fixed satellite system may need to be configured to support the air interface used by the mobile satellite radiotelephone system.
Alternatively, if different air interfaces are used by the fixed and mobile satellite radiotelephone systems, the fixed retransmitting station can perform a translation from one air interface to the other, for example, by demodulation and remodulation. The fixed retransmitting station then becomes a regenerative repeater which reformats communications channels as well as control channels. However, if the mobile and fixed systems both use substantially the same air interface, then the fixed retransmitting station can function as a non-regenerative repeater.
One embodiment may use the simplest fixed retransmitting station by having the fixed and mobile systems both utilize the same air interface standard. Alternatively, the fixed system is configured to support the mobile system air interface even though the fixed system may be using another air interface for fixed radiotelephone service.
FIG. 3 is another prior art system as shown in U.S. Pat. No. 5,995,832. FIG. 3 provides an overview of a communications system 310 showing the functional inter-relationships of the major elements. The system network control center 312 directs the top level allocation of calls to satellite and ground regional resources throughout the system. It also is used to coordinate system-wide operations, to keep track of user locations, to perform optimum allocation of system resources to each call, dispatch facility command codes, and monitor and supervise overall system health. The regional node control centers 314, one of which is shown, are connected to the system network control center 312 and direct the allocation of calls to ground nodes within a major metropolitan region. The regional node control center 314 provides access to and from fixed land communication lines, such as commercial telephone systems known as the public switched telephone network (PSTN). The ground nodes 316 under direction of the respective regional node control center 314 receive calls over the fixed land line network, encode them, spread them according to the unique spreading code assigned to each designated user, combine them into a composite signal, modulate that composite signal onto the transmission carrier, and broadcast them over the cellular region covered.
Satellite node control centers 318 are also connected to the system network control center 312 via status and control land lines and similarly handle calls designated for satellite links such as from PSTN, encode them, spread them according to the unique spreading codes assigned to the designated users, and multiplex them with other similarly directed calls into an uplink trunk, which is beamed up to the designated satellite 320. Satellite nodes 320 receive the uplink trunks, frequency demultiplex the calls intended for different satellite cells, frequency translate and direct each to its appropriate cell transmitter and cell beam, and broadcast the composite of all such similarly directed calls down to the intended satellite cellular area. As used herein, “backhaul” means the link between a satellite 320 and a satellite node control center 318. In one embodiment, it is a K-band frequency while the link between the satellite 320 and the user unit 322 uses an L-band or an S-band frequency.
A “node” is a communication site or a communication relay site capable of direct one or two-way radio communication with users. Nodes may include moving or stationary surface sites or airborne or satellite sites.
User units 322 respond to signals of either satellite or ground node origin, receive the outbound composite signal, separate out the signal intended for that user by despreading using the user's assigned unique spreading code, de-modulate, and decode the information and deliver the call to the user. Such user units 322 may be mobile or may be fixed in position. Gateways 324 provide direct trunks that is, groups of channels, between satellite and the ground public switched telephone system or private trunk users. For example, a gateway may comprise a dedicated satellite terminal for use by a large company or other entity. In the embodiment of FIG. 3, the gateway 324 is also connected to that system network controller 312.
All of the above-discussed centers, nodes, units and gateways are full duplex transmit/receive performing the corresponding inbound (user to system) link functions as well in the inverse manner to the outbound (system to user) link functions just described.
Referring now to FIG. 4, which is another embodiment as shown in U.S. Pat. No. 5,995,832, a block diagram of a communications system 440 which does not include a system network control center 312 is presented. In this system, the satellite node control centers 442 are connected directly into the land line network as are also the regional node control centers 444. Gateway systems 446 are also available as in the system of FIG. 3, and connect the satellite communications to the appropriate land line or other communications systems. The user unit 322 designates satellite node 442 communication or ground node 450 communication by sending a predetermined code. Alternatively, the user unit could first search for one type of link (either ground or satellite) and, if that link is present, use it. If that link is not present, use the alternate type of link.
The specification of U.S. Pat. No. 5,995,832 states that “[m]easures incorporated in the invention to maximize bandwidth utilization efficiency include the use of code division multiple access (CDMA) technology which provides an important spectral utilization efficiency gain and higher spatial frequency reuse factor made possible by the user of smaller satellite antenna beams. In regard to power efficiency, which is a major factor for the satellite-mobile links, the satellite transmitter source power per user is minimized by the use of forward-error-correcting coding, which in turn is enabled by the above use of spread spectrum code division multiple access (SS/CDMA) technology and by the use of relatively high antenna gain on the satellite.”
The specification of U.S. Pat. No. 5,995,832 also states that “[i]n a system in accordance with the invention, the cluster size is one. That is, each cell uses the same, full allocated frequency band. This is possible because of the strong interference rejection properties of spread spectrum code division multiple access technology (SS/CDMA).” With regard to determining the position of user units 322, the specification of U.S. Pat. No. 5,995,832 states that “[a]ccurate position determination can be obtained through two-dimensional multi-lateration. Each CDMA mobile user unit's transmitted spreading code is synchronized to the epoch of reception of the pilot signal from its current control site, whether ground or satellite node.”
However, in contrast to the prior art systems described, for example, in FIGS. 1–4, the present invention does not utilize in one embodiment frequency translation between fixed and mobile systems. In addition, the present invention provides, for example, a robust satellite-terrestrial frequency assignment and/or reuse scheme in another embodiment. Further, the present invention optionally utilizes a first frequency as a downlink frequency between a satellite and a first fixed and/or mobile user terminal and as an uplink frequency between a second fixed and/or mobile user terminal and a base station, and a second frequency as an uplink between the first fixed and/or mobile user terminal and the satellite and as a downlink between the base station and the second fixed and/or mobile user terminal. Finally, the present invention is not limited, for example, to the use of CDMA technology. Other advantages and features of the invention are described below, that may be provided independently and/or in one or more combinations.